The speaker size for the 2006 ford mustang gt will be the same as the base models of this vehicle. the gt upgrade focuses on upgrades to the engine and not to the speakers or sound system. the front doors will be equipped with 6x8" speakers and additionally they will each come equipped with an 8-inch subwoofer. the rear deck corners are going to have the same 6x8" speakers as the front doors but they will not come equipped with a subwoofer. the standard speakers in this vehicle should be powerful for most. while there are not many speakers this is a small vehicle so you don't need too many for it to pack a punch.
The 2006 ford mustang gt speakers measure 6x8" on the front door and rear deck corners. the front door woofers are 8".